Hello Donghee,
I have a present for you 
TFile* f = new TFile(inFile.Data()); // the sim file you want to analyse
TTree* t=(TTree*)f->Get("cbmsim");
FairMCEventHeader* evthead;
t->SetBranchAddress("MCEventHeader.", &evthead);
for (Int_t j=0; j<nEvents && j<t->GetEntriesFast(); j++)
{
t->GetEntry(j);
if(verbosepoints) cout<<"Event No "<<j<<endl;
else if (!(j%100)) cout <<"Event No "<<j<<endl;
cout<<"GetRunID() "<<evthead->GetRunID()<<endl;
cout<<"GetEventID() "<<evthead->GetEventID()<<endl;
cout<<"GetX() "<<evthead->GetX() <<endl;
cout<<"GetY() "<<evthead->GetY() <<endl;
cout<<"GetZ() "<<evthead->GetZ() <<endl;
cout<<"GetT() "<<evthead->GetT() <<endl;
cout<<"GetNPrim() "<<evthead->GetNPrim() <<endl;
}// end for j (events)
Be reminded of the Branch name (thaks Tobias) and that the Event header is an object directliy inside the tree, like all the TClonesArrays.
Kind regards,
Ralf